Traditionally seen as harbingers of death or symbols of evil, the crow family of birds are often considered to be pests.

But a French park has taken advantage of their innate intelligence and trained the birds to pick up litter.

Six rooks have been taught to patrol a popular historical theme park in the western Vendée region and clear up cigarette ends and other rubbish in return for food.

When a bird deposits a cigarette butt or a piece of rubbish in a box, a morsel of food is dispensed as a reward. Some rooks have already started work.

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Rooks are considered "particularly intelligent" members of the crow family, according to Nicolas de Villiers, head of the Puy du Fou park.

"In an affectionate, supportive atmosphere, they like to communicate with humans and establish a relationship through play," he said.

"The goal is not just to clear up, because visitors are generally careful to keep things clean, but also to demonstrate that nature itself can teach us to take care of the environment."

The rooks are quick workers, able to fill a bucket with rubbish in less than 45 minutes, Villiers said.

A park falconer hatched the idea of putting them to work. Christophe Gaborit, who is accustomed to training birds of prey, said: "They were motivated by the reward and they soon understood how the game worked."

Crows can damage crops by eating seeds and grain. They are noisy and have been known to attack cars and wrench off windscreen wipers.

Yet they are highly sociable and so intelligent that some biologists say they are as smart as the average 7-year-old child.

They make tools, tearing strips from leaves and using them to winkle bugs out of tree trunks. Experiments have shown them to be capable of solving relatively complex puzzles. They will bend wire to fashion hooks, even if they have never encountered wire before.

Scientists tested whether the rooks were really clever enough to pick up the litter by placing them in a room with a worm in a tube of water just below pecking distance, with a small pile of pebbles nearby. The birds dropped the pebbles into the water until the worm came within reach.

They avoided objects that would float in the water and those too large for the container, demonstrating a grasp of volume displacement acquired by children between the ages of 5 to 7.

Frédéric Jiguet, an ornithologist at the National Museum of Natural History in Paris, said crows are capable of recognising individual human faces.

"I captured a series of crows in a park, tagged them and released them. They didn't like it. Many weeks later, I returned to the park with colleagues, and the crows I had tagged attacked me but ignored the others."

Villiers said visitors to his theme park, the second most visited in France after Disneyland Paris, were entranced by the birds' performance as cleaners.